1. Field of the Invention
The present invention relates to a method of stabilizing shapes of optical elements and an optical apparatus employing the same. More particularly, the present invention relates to a method of stabilizing shapes of optical elements which prevents shape changes caused by local temperature changes and which is intended for use with synchrotron radiation light or high-intensity light beams which are widely used for physics and chemistry research, analysis apparatuses, manufacturing apparatuses or the like, and to an optical apparatus employing the same.
2. Description of the Related Art
In recent years, a light source which emits synchrotron radiation (SR) light or high-intensity light beams, such as excimer laser beams, has been developed. Optical apparatuses using such a light source for physics and chemistry research, analysis apparatuses, manufacturing apparatuses or the like have lately attracted attention, and a lot of research and development of these apparatuses has been performed.
Generally, optical apparatuses require various types of optical elements for the purposes of reflection, transmission, light condensation, diffraction, spectrophotometry, polarization, image formation or the like. Since the intensity of light beams used is high in these optical apparatuses, which use, in particular, a high-intensity light source, phenomena such as deformation, performance deterioration, irradiation damage, or destruction of optical elements occur.
An example of such high intensity light beams concerns the SR light technology field, where the radiation power from a light source has recently reached the order of kilowatts as the result of advancements made in the technology regarding what is commonly called insertion type light sources, such as multi-pole wigglers or undulators.
Radiation ranging from X-rays to electromagnetic waves in vacuum ultraviolet rays are often used regarding radiation lengths from a light source. To prevent attenuation in the atmosphere, in most cases, the optical elements are installed in a vacuum vessel or a vacuum beam line. As a result, heat radiation by conduction or convection to the atmosphere does not occur in optical elements placed in a vacuum. Thus, there is a tendency for the temperature of optical elements to increase much more than when they are placed in the atmosphere.
To be specific, in a semiconductor exposure apparatus which uses synchrotron radiation light or high-intensity illumination light from an excimer laser or the like, heat strain caused by temperature changes or temperature distribution changes of mirrors or lenses which reflect, converge, and enlarge illumination light, or caused by reticles or masks, becomes a major obstacle to the improvement of accuracy of the semiconductor exposure apparatus. In particular, when electromagnetic waves, such as synchrotron radiation light or vacuum ultraviolet rays, are used, optical systems, such as mirrors or lenses, and masks are generally placed in a vacuum chamber or a pressure reduction chamber in order to prevent the attenuation of the energy of illumination light. If members to be illuminated, such as mirrors, lens, or masks, are heated by illumination light, the temperature increases considerably because there is hardly any conduction by atmosphere gas or heat radiation by convection. Heat strain caused by temperature changes or changes in temperature distribution considerably changes the distribution of intensity of illumination light, causing exposure irregularities.
For this reason, various methods for cooling the above-mentioned members to be illuminated by using a water cooling jacket or the like have been developed. An example thereof is described in "Rev. Sci. Instrum. 60, 1493 (1989), T. Oversluizen, et al.".
However, in an optical apparatus using a light source which emits high-intensity light beams, even if optical elements are cooled, the temperature of the surfaces of the optical elements continues to vary usually by several to several tens of degrees. As a result, the shapes of the optical elements slightly change their forms, and there are some cases in which the optical performance of the optical apparatuses deteriorates.
For example, in a projection exposure apparatus for transferring fine patterns for use in the manufacturing of semiconductor devices, it is required that temperature variations of optical elements, such as mirrors, be controlled to approximately 1/100.degree. C. However, in the prior art, the cooling of optical elements alone is not sufficient to control the temperature variations of the optical elements to approximately 1/100.degree. C.